UNIVERSITY of NEW SOUTH WALES School of Civil Engineering M ASSESSMENT of NETWORK MODELS for FLOOD FORECASTING Name

UNIVERSITY OF NEW SOUTH WALES School of Civil Engineering M ASSESSMENT OF NETWORK MODELS FOR FLOOD FORECASTING Name: T. A. Malone Award: Master of Engineering Science Year of Submission: 1989 Supervisor: Dr I. Cordery UNIVERSITY OF N.S.W. - 3 OCT 1990 LIBRARY 1. STUDENT’S DECLARATION a. This is to certify that I, ....S...... ..............................., being a candidate for the degree of Master of Engineering Science am fully aware of the policy of the University relating to the retention and use of higher degree projects, namely that the University retains the copies of any thesis submitted for examination, "and is free to allow the thesis to be consulted or borrowed. Subject to the provision of the Copyright Act (1968) the University may issue the thesis in whole or in part, in photostat or microfilm or other copying medium". I also authorize the publication by the University Microfilms of a 600 word abstract in Dissertation Abstracts International (D. A. I.). b. I hereby declare that none of the work in this project has been submitted to any other institution for the award of a higher degree. 2 SUPERVISOR’S CERTIFICATION I certify that this project has been completed under my supervision and is in my opinion in a form suitable for examination as part of the requirement for admission to the degree of Master of Engineering Science. AN ASSESSMENT OF NETWORK MODELS FOR FLOOD FORECASTING TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES Section Page 1. INTRODUCTION 1 2. CATCHMENT DESCRIPTION 5 2.1 Tweed River to Murwillumbah 5 2.2 Wilsons River to Lismore 8 2.3 Macleav River to Kempsev 11 3. DATA COLLATION AND PREPARATION 15 3.1 Rainfall Data 16 3.2 River Data 18 3.3 Selection of Flood Events 19 3.4 Baseflow Separation 21 3.5 Rainfall Loss Model 23 3.6 Rainfall Distribution 24 4. CATCHMENT MODELS 26 4.1 Unit Hvdrograph 27 •4.2 RORB 27 4.3 WBNM 32 4.4 Model Comparison 36 4.4.1 Unit Hvdrograph vs Network Models 36 4.4.2 RORB vs WBNM 37 5. METHODOLOGY 39 5.1 Model Calibration 39 5.1.1 Rainfall Losses 39 5.2.2 Unit Hvdrograph 41 5.1.3 Network Models 44 5.2 Results 48 6. DISCUSSION 51 6.1 Rainfall Distribution and Losses 51 6.2 Model Performance 53 6.3 General Discussion 59 7. CONCLUSIONS AND RECOMMENDATIONS 62 REFERENCES APPENDICES A - Rating Tables B - Wilsons River to Lismore Unit Hydrograph Derivation C - Macleav River to Kempsev Typical RORB & WBNM Calibrations D - Summary of Results List of Tables Page 2.1 Flood Level Classifications 8 3.1 Floodwarn Networks 16 3.2 Selected Flood Events 20 5.1 Initial and Continuing Losses 40 5.2 Unit Hvdrograph Peak Times 42 5.3 Three Hour Unit Hydrographs 43 5.4 Model Calibration Parameters 46 5.5 Summary of Model Results 49 List of Figures Page 2.1 Tweed River - Catchment Map 6 2.2 Wilsons River - Catchment Map 9 2.3 Macleay River - Catchment Map 12 3.1 Baseflow Separation 22 3.2 Initial Loss - Continuing Loss Model 23 4.1 Tweed River - RORB Network 28 4.2 Wilsons River - RORB Network 29 4.3 Macleay River - RORB Network 30 4.4 Tweed River - WBNM Network 34 4.5 Macleay River - WBNM Network 35 -5.1 Tweed River - RORB Parameter Indifference Curves 45 6.1 Tweed River - Model Results 57 ABSTRACT Two similiar non-linear network models have been applied to three catchments in coastal NSW to examine their potential for use in operational flood forecasting. Network models have the potential to make direct allowance for non-uniform catchment response to input rainfall. The two models, RORB and WBNM, were applied under operational forecasting conditions to data for 7 to 8 flood events on each catchment and their ability to reproduce the observed hvdrographs was compared with the unit hydrograph approach which is curently used for flood forecasting in each catchment. The results indicate that the determination of excess rainfall is probably more critical than the type of model used to calculate the direct runoff hydrograph. There appears to be little difference between the abilities of the unit hvdrograph and the network models to reproduce the observed hvdrographs on the three catchments which ranged in size from 630 square kilometres to 10,000 square kilometres. However overall, the network models offer a number of advantages over the unit hydrograph for flood forecasting. AN ASSESSMENT OF NETWORK MODELS FOR FLOOD FORECASTING 1. INTRODUCTION Under the Meteorology Act of 1906, the Commonwealth Meteorologist was given the responsibility, amongst other things, for 'the display of .... flood signals' and in that year the Bureau commenced the issue of generalised flood warnings. However, it was not until a series of highly damaging floods in NSW in the mid-1950s that a Hydrometeorological Service was established in the Bureau with one of its main aims being to provide systematic flood forecasts. Flood forecasts provided by the Bureau include both qualitative and quantitative forecasts. Qualitative forecasts are statements of expected occurences of a flood event in the broad descriptive classes of minor, moderate and major flooding. Quantitative forecasts are the ultimate aim of the Bureau of Meteorology flood forecasting service providing height predictions at key river gauges in a catchment, with a pre-determined degree of accuracy and lead time. The classification of flooding at a key river gauge is based on the following definitions: Minor Flooding - This causes inconvenience such as cutting of minor roads and submergence of low level bridges and makes the removal of river pumps necessary. Minor flooding causes little inundation of land adjacent to the river. 1 Moderate Flooding - This causes inundation of low ly ing areas requiring the removal of livestock and the evacuation of isolated houses. Main traffic bridges may be closed. Major Flooding - This causes inundation of large areas, isolating towns and cities. Major disruption occurs to road and rail traffic and other communications. Evacuation of many houses and business premises may be required. The most important requirement of a flood forecasting system is to provide the maximum lead time of a river reaching a specified height at a specified time at a particular location. Based upon this requirement, different types of flood forecasting systems are employ ed on different sized catchments. In smaller catchments these tend to be rainfall based methods whilst in the larger catchments river routing techniques tend to be employed. Often a combination of these techniques is used to predict downstream flood heights. When the Bureau commenced providing quantitative flood forecasts in small river basins, the unit hydrograph was the only rainfall based hydrologic model readily available. Often during floods, real time rainfall information available was limited to a relatively small number of stations which reported at three hourly intervals to the 2 Bureau. Operational requirements dictated that flood predictions, based on the latest observations from these stations, were broadcast to the public every three hours. Frequently flood forecasts were required for more than one river at a time when computing ability and facilities were very limited. Under these conditions, the unit hydrograph proved to be ideal as it was robust and simple to use. As a consequence, approximately 50 were derived by the Bureau for flood forecasting operations in NSW. The advent of computers and their increasing availability has seen the more widespread use of rainfall routing or network models, especially in probabilistic hydrology. As yet, they have found limited use in operational hydrology. The purpose of this study is to determine if network models give more accurate and consistent results than unit hydrographs when used to forecast floods of different magnitudes. Two similar non-linear network models, RORB and WBNM, will be compared with the unit hydro graph on three catchments under similar conditions to operational use. The quantity and quality of the data used and the assumptions made during this investigation are similar to those which occur in operational flood forecasting. This important underlying aspect of the study cannot be emphasised strongly enough. 3 The three catchments selected for investigation are ir coastal NSW and are of varying sizes. The Tweed River tc Murwi 1 lurnbah has a catchment area of about 630 square kilometres, the Wilsons River to Lismore an area of 140C square kilometres and the Macleay River to Kempsey is mucl larger with a catchment area of 10,000 square kilometres. Flooding is a problem in each of these catchments and unit hydrographs have been used regularly in past floods to forecast heights at critical locations with varying success. 4 2. CATCHMENT DESCRIPTION 2.1 TWEED RIVER TO MURWILLUMBAH The Tweed River to Murwillurnbah, located on the northern NSW coast near the Queensland border, comprises the catchment areas of the Tweed River and the Oxley River and the smaller catchment of Dunbible Creek. The total catchment area to Murwillurnbah, shown on Figure 2.1, is 630 square kilometres. Mt Warning, a 1150 metre remnant core of a volcano, dominates the catchment with a symmetrical drainage pattern emitting from it. The northern arm, the Oxley River, rises in the rugged MacPherson Ranges at elevations between 900 and 1100 metres. A number of small tributaries converge before flowing eastward and joining the Tweed arm about 7 kilometres upstream of Murwillurnbah. The southern arm is the Tweed River which rises in the mountain range in the south between elevations of 600 to 900 metres. Various tributaries drain northward before joining the main watercourse which then flows eastward to Murwillurnbah. Dunbible Creek which drains the eastern slopes of the valley joins the Tweed River just upstream of Murwillurnbah.

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